Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 171053

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
553 AM CDT Tue Oct 17 2017

220 AM CDT

Through Wednesday...

Mostly sunny and mild fall weather will be
the story for the upcoming days. High pressure stretches from
central Texas northeast through the Ohio valley and into the mid
Atlantic. The high will slowly become centered a bit farther east
each day. Meanwhile, a series low pressure systems will slide by
north of the Canadian border. One today, and a slightly stronger
system on Wednesday. The sensible weather result will be breezy
southwest winds, though plentiful sunshine. Highs will be in the
ballpark of 70 degrees each day after the cooler nights.



220 AM CDT

Thursday through Monday...

The stronger northern low will send a weak cold front toward the
area Wednesday night into Thursday. Impacts will be fairly
negligible, but we will see a minor cooldown, and winds will
slacken and veer to WNW, before weak high pressure spreads east of
Lake Michigan on Thursday afternoon. This will bring some brief
onshore flow, and thus temps may dip a few degrees. We may also
get a few clouds with this too.

The pattern Friday reverts to that of earlier in the week, with the
high over the Ohio valley/mid Atlantic region, but the low will now
be ejecting from a deep/broad western upper level trough axis. With
seasonally mild overnight lows, Friday highs should springboard into
the 70s area wide. Most of Saturday will be dry as we await the
arrival of the upper trough, but expect clouds to thicken. Still, it
will be mild with 70s.

The party will come to an end Saturday night into Sunday as the
upper trough will move through. While the forcing is broad and
stretched out, a feed of 60+ dewpoints will advect in ahead of it on
a strong low level jet. Upper forcing will be augmented by a
strengthening upper jet. There are some timing differences in the
longer range guidance as to how quickly the moisture from the
Gulf of Mexico gets this far north which impact the specific 6
hour PoP forecasts, but all indicate a favorable ramp up in
precipitation chances by Saturday night or more so Sunday morning.
With plentiful clouds, instability will be tempered, but there
could be a few t- storms. Colder air will return behind this
systems surface front as potent northwest flow will setup across
the northern tier of the country. Even colder air may be on the
horizon in far-out model land.



For the 12Z TAFs...

Winds will be the only aviation concern today as the surface
gradient strengthens over the region. Expect winds to become
gusty out of the southwest this morning. Wind gusts into the low
to mid 20 KT range can be expected across the area by early this
afternoon. These gusty winds will abate by sunset. Otherwise,
expect VFR conditions through the period. Gusty south-
southwesterly winds will return on Wednesday.



259 AM CDT

The main weather concern over Lake Michigan this week is the
increasing potential for a period of high end southerly gales over
the northern half of the lake on Wednesday and Wednesday evening.

An expansive area of high is expected to become extended from the
Lower Ohio Valley to the Mid-Atlantic today, and then remain
dominate here through much of the week. Meanwhile, a couple of
storm systems will shift eastward across Canada through mid week.
The associated surface pressure gradient over the lake will
continue to be favorable to support a prolonged period of stout
southwest winds through mid to late week.

The first storm system, is currently shifting eastward across
northern Ontario. This system is currently supporting
southwesterly winds to 30 KT. These wind speeds will gradually
ease through the day as this system shifts into northwestern
Quebec later today.

The second storm system is expected to be the main story of the
week, however, as it is forecast to be a much more potent area of
low pressure. As such, this will set up a very strong gradient
over the lake on Wednesday and Wednesday night. During this period
high end south-southwesterly gales of 40 to 45 kt will be
probable, especially over the northern half to 2/3rds of the
lake. With this in mind, the current gale watch will likely need
to be converted to a gale warning later today.

Yet another period of enhanced southerly winds look possible late
Friday into Saturday as another storm system shifts northeastward
into south central Canada. Wind speeds with this event could
again approach gale force.



LM...Small Craft Advisory...nearshore waters
     until 7 PM Tuesday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.